Cell phones and other radio frequency (RF) devices often utilize a film bulk acoustic wave resonator (FBAR) for filtering RF signals. The FBAR resides on a substrate such as a semiconductor or glass substrate and includes a thin film of a piezoelectric material such as zinc oxide (ZnO) or aluminum nitride (AlN) that is sandwiched between metal electrodes. The RF signal drives the electrodes and will excite a resonant excitation in the piezoelectric layer at the fundamental resonant frequency for the FBAR that depends upon the thickness of the piezoelectric layer.
A conventional FBAR 100 is shown in FIG. 1. A piezoelectric layer 102 is sandwiched between upper and lower metal layers/electrodes 104 and 106. The resonant frequency for FBAR 100 is determined by a total thickness d across lower electrode 106, the piezoelectric layer 102, and the upper electrode 104. An RF signal applied across electrodes 104 and 106 produces an electric field within piezoelectric layer 102 that induces a bulk acoustic wave. In order to maintain high quality performance, the bulk acoustic wave should be isolated from a substrate 108. In one configuration, FBAR 100 is unsupported, i.e., there is an “air gap” 110 between FBAR 100 and substrate 108. The acoustical isolation from air gap 110 prevents acoustic energy leakage from the device into substrate 108.
Since the resonant frequency depends upon the thickness d, an RF filter operating in several frequency bands requires multiple FBARs each having an appropriate thickness to provide the desired resonant frequencies. But the deposition of the piezoelectric layer for an FBAR is performed under vacuum using physical vapor deposition. If the vacuum is broken so that additional mask steps can be performed followed by additional piezoelectric layer depositions to produce different piezoelectric layer thicknesses, the piezoelectric layer quality is compromised. A conventional multi-band RF filter implemented using FBARs thus requires separate FBARs each having their own substrates so that the deposition of the piezoelectric layer thickness can be individually tuned. But the usage of multiple FBAR devices increases costs and manufacturing complexity. In another attempt to produce multiple FBARs on a single substrate for a multi-band RF filter, a deposited single-thickness piezoelectric layer may be etched using multiple etching and masking steps to provide various piezoelectric layers of different thicknesses on the same device. But the etching of the piezoelectric layer is problematic in that the electrode-facing surfaces become too rough, which reduces the quality for the resulting FBAR devices.
Accordingly, there is a need in the art for an improved multi-FBAR device.